1. Field of the Invention
The present invention relates to a power unit for driving left and driven parts for propelling a transport machine in a manner capable of traveling forward in a straight line or in a manner capable of turning.
2. Description of the Related Art
Conventionally, as a power unit of this kind, one disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H11-91524 is known. This power unit is for driving left and right drive wheels of a vehicle, and is provided with first and second planetary gear units, first and second electric motors, and brakes. The first planetary gear unit has a sun gear and a carrier thereof connected to a first electric motor and the left drive wheel, respectively, and the second planetary gear unit has a sun gear and a carrier thereof connected a second electric motor and the right drive wheel, respectively. Further, the first planetary gear unit and the second planetary gear unit have respective ring gears connected to each other, and each of the aforementioned brakes holds the associated ring gear unrotatable when it is on, and permits the rotation thereof when it is off.
In the conventional power unit, by turning on the brakes, the ring gears are held unrotatable, and at the same time, the first and second electric motors are caused to rotate normally or inversely together with the respective associated sun gears, whereby the left and right drive wheels are rotated normally or inversely together with the respective associated carriers, which causes the vehicle to move forward or rearward. Further, by turning off the brake to permit the rotations of the ring gears, causing the first electric motor to rotate normally and causing the second electric motor to turn inversely, the speed of the left drive wheel is increased with reference to the rotational speed of the ring gear, and the speed of the right drive wheel is reduced, whereby a right turn of the vehicle is assisted. Further, in the above situation, by causing the first electric motor to rotate inversely and the second electric motor to rotate normally, a left turn of the vehicle is assisted.
However, as described hereinabove, in the conventional power unit, the brakes are indispensable for causing the vehicle to move forward or rearward. Therefore, the provision of the brakes results in an increased size of the power unit and increased manufacturing costs of the same.